The purpose of this research is to examine the ultrastructure of cellular differentiation in the human fetal ovary and testis. With the use of freeze-fracture, lanthanum tracer and standard electron microscopic techniques, cell membrane interrelationships in the developing gonads, including the special types of intercellular junctions present, will be studied in detail. Beginning with gonadal differentiation at 6-7 weeks gestation, this investigation will extend as far as the 20 week period. Examination will be made of the types of intercellular junctions present between germ cells and granulosa cells and between adjacent granulosa cells in the ovarian cortex prior to and during follicle formation. The pattern of junctional interrelationships in the ovarian surface epithelium and between interstitial cells in the fetal ovary will also be studied. Membrane relationships in the seminiferous cords of the fetal testis will be examined to determine the types of junctions present between germ cells and Sertoli cells and between adjacent Sertoli cells. The cell membrane characteristics and interrelationships of peritubular myoid cells will be studied. Particular attention will be directed toward the appearance and arrangement of specialized gap junctions between fetal Leydig cells. Determination will be made of the types of relationships exhibited by these cells during the period in which they undergo differentiation and proliferation associated with active testosterone production and in the subsequent period of regression.